1. Field of the Invention
The present invention relates to a process for the oxidation of a metal from a lower to a higher valence state. More particularly, the present invention provides a process for the oxidation of the metal in a particular type of aeration flotation apparatus.
2. Description of the Prior Art
There are a number of chemical processes requiring efficient and economical methods of oxidizing multivalent metals and compounds thereof from lower to higher valence states. One of such processes involves the regeneration of a metal, such as an iron chelate solution used in the process of removing hydrogen sulfide from gases, e.g., see EPA No. 82.306861.4, filed Dec. 22, 1982, published on Aug. 31, 1983.
The removal of hydrogen sulfide from gaseous streams, such as the waste gases liberated in the course of various chemical and industrial processes, for example, in the pulping of wood, natural gas and crude oil production and in petroleum refining, has become increasingly important in combating atmospheric pollution. Hydrogen sulfide-containing gases not only have an offensive odor, but such gases may cause damage to vegetation, painted surfaces and wildlife, as well as constitute a significant health hazard to humans. Government regulations have increasingly imposed continuously lower tolerances on the content of hydrogen sulfide which can be vented to the atmosphere, and it is now imperative in many localities to remove virtually all of the hydrogen sulfide under the penalty of an absolute ban an continuing operation of commercial plants which produce the hydrogen sulfide-containing gaseous streams.
The quantities of hydrogen sulfide in process gas streams are normally not very high. U.S. Pat. No. 3,071,433, dated Jan. 1, 1964 to Dunn, indicates that the stack gases obtained in the concentration of blank liquor, the waste pulping liquor of the Kraft pulping process, contain from 500 to 2000 parts per million (ppm) of hydrogen sulfide. However, the odor of hydrogen sulfide can be detected by humans at a concentration of approximately 0.01 ppm. Consequently, an extremely efficient process for the removal of hydrogen sulfide is required to eliminate small amounts of noxious hydrogen sulfide from process gases.
Carbon dioxide may also be present with hydrogen sulfide as a contaminant of various gases, e.g., those obtained from well casings, combustion floods, geothermal steam, or tank vapors. Often, it is not only desirable to remove H.sub.2 S from such gases, but to selectively remove H.sub.2 S and not remove the carbon dioxide.
One well known method in the art for removing hydrogen sulfide from a gas stream involves contacting the gas stream with caustic soda, which scrubs the acid gases from the gas stream. U.S. Pat. No. 2,747,962 to Heitz et al provides a method whereby acid gases, such as hydrogen sulfide, are removed selectively from a gas stream also containing carbon dioxide using an alkaline liquid, such as caustic soda, to remove the acid gases. The absorption of the CO.sub.2 is much slower than the absorption of H.sub.2 S, and thus the absorption of CO.sub.2 can be prevented by maintaining a very short contact time (0.01-0.02 second) between the gas stream and alkaline liquid. However, a disadvantage of this process is that when the alkaline liquid is regenerated by heating to about 270.degree. F., H.sub.2 S is produced, and thus the H.sub.2 S disposal problem is not solved but merely postponed. Alternatively, a relatively toxic sodium sulfide solution must be disposed of.
It is also known to effect the removal of hydrogen sulfide in an oxidation-reduction system by contacting the hydrogen sulfide-containing gas stream with a solution of a polyvalent cation (such as iron) complexed with a chelating agent (such as ethylenediaminetetraacetic acid or sodium salt thereof). In such a process, iron in the ferric state oxidizes the hydrogen sulfide to sulfur, the iron is reduced to the ferrous state, and the solution is regenerated by aeration to convert the iron back to the ferric state. The sulfur is recovered from the solution by froth flotation.
For example, U.S. Pat. No. 4,036,942 to Sibeud et al discloses a process for removing hydrogen sulfide and alkyl mercaptans from fluid streams by reacting the fluid streams with oxygen in the presence of a metal amino acid chelate in an aqueous solution containing an amine, resulting in the conversion of hydrogen sulfide to sulfur and alkyl mercaptans to dialkyldisulfides, and separating these from the aqueous metal chelate solution. However, the presence of oxygen in the reactants is disadvantageous in that it results in the conversion of some of the sulfur to sulfates and thiosulfates. Furthermore, the reaction requires a relatively long contact time between the metal chelate solution and the hydrogen sulfide-containing gas stream, such that if carbon dioxide is also present in the gas stream, the required contact time results in the absorption of carbon dioxide into the reaction solution, thus lowering the pH and lowering the efficiency of the reaction.
U.S. Pat. No. 4,009,251 to Meuly also discloses a process for removing hydrogen sulfide and alkyl mercaptans from gaseous streams by oxidizing the hydrogen sulfide to sulfur substantially without the formation of sulfur oxides, in the presence of a metal chelate catalyst solution and a soluble alkali metal, or alkaline earth metal or ammonium or amine salt of an acid having a pK within the range of about 1.2 to about 6. The spent metal chelate catalyst solution is then regenerated with oxygen. The alkyl mercaptans are oxidized to dialkyldisulfides under the same conditions. Meuly attempts to eliminate the oxidation of hydrogen sulfide through to sulfur oxides by the addition of the above-described acid salts. Such addition to the metal chelate catalyst solution is required, since Meuly reacts the hydrogen sulfide-containing gas stream with oxygen and recognizes that sulfur oxides may be formed by such a reaction mixture. Furthermore, the process of the above patent requires relatively long contact times for oxidation, and thus when carbon dioxide is present in the hydrogen sulfide-containing gas stream, the relatively contact time also results in the absorption of CO.sub.2 and the consequential reduction in the pH of the solution and reduction in the efficiency of the system.
In many other industrial/chemical processes it is important to oxidize a multi-valent metal from a lower to a higher valence state by contacting the solution thereof with an oxygen-containing gas.